1. Field of the Invention
The present invention relates to an ophthalmic positioning apparatus which is used in, for example a noncontact type tonometer and which projects a positioning index onto an eye to be examined and performs positioning (alignment control and/or operation distance control) for an ophthalmic instrument with respect to the eye in accordance with an index image reflected by the eye.
2. Description of the Prior Art
In a conventional ophthalmic instrument, exemplified by a noncontact type tonometer using air pulses, air pulses are ejected onto the cornea of an eye to be examined along the axial direction of the instrument. Corneal deformation-sensing light-projecting and light-receiving systems are inclined substantially symmetrically about the axial direction of the ophthalmic instrument. Eye pressure is determined by measuring the time required to cause the light-receiving system to generate a maximum output, that is, the time required to flatten the cornea. However, when the distance between the ophthalmic instrument and the vertex of the cornea does not coincide with a predetermined operation distance or when the vertex of the cornea and the horizontal eye axis do not fall in line with the axis of the ophthalmic instrument, measured values are not accurate. For this reason, a conventional noncontact type tonometer has an optical positioning system to adjust the distance between the tonometer and the vertex of the cornea so that it coincides with the predetermined distance, or to adjust the axis of the tonometer so that it falls in line with the vertex of the cornea. A doctor performs alignment and operation distance adjustment and sends out air pulses so as to clearly form a positioning index image at the center of the field of view in a finder. A noncontact type tonometer with such an optical positioning system is described in U.S. Pat. No. 3,756,073. Although this optical system can perform an operation distance adjustment to clearly focus the cornea-reflected image of the positioning index, two index images of the same size appear at two points with different operation distances and cannot be clearly distinguished from each other. Furthermore, the two points with different operation distances are only slightly separated from each other. In the conventional positioning apparatus, as a result, an index having a position which does not coincide with a normal operation distance can be mistakenly chosen, thus resulting in an inaccurate measurement.